Modular system for unattended energy generation and storage

ABSTRACT

An apparatus and method for supplying energy to a load includes an energy recharge unit, an energy storage unit, an energy converter connected to the energy recharge unit, the energy converter being capable of transferring energy at a power level from the energy recharge unit to an output node, the power level being determined by a power transfer controller, and a bi-directional energy converter connected to the energy storage unit and to the output node. The bi-directional energy converter is capable of converting energy of varying voltages from the energy storage unit to energy of varying current levels to supplement the transferred energy with energy from the energy storage unit so as to maintain a constant voltage on the output node. The bi-directional energy converter is capable of converting the transferred energy to provide charging energy to the energy storage unit when the transferred energy exceeds a demand level of the load while maintaining the constant voltage at the output node.

GOVERNMENT RIGHTS

This invention was made with government support under Contract No.H92222-06-C-0002 awarded by the Department of Defense. The U.S.government has certain rights in the invention.

FIELD

The invention relates, generally, to energy generation and storagesystems and, more particularly, to a modular system for unattendedenergy generation and storage.

BACKGROUND

Remote sensors, isolated communication devices, distributed wirelessnetworks, and a host of other unattended electrically-operated systemstypically require for operation minimum levels of electrical energy.These unattended electrically-operated systems are generally unreachableby conventional electric power or utility grids. As such, alternativeenergy or power sources such as, for example, solar energy, wind energy,and geothermal energy, have increasingly been relied upon to supplytheir required electrical energy.

While a few of these unattended electrically-operated systems aregeographically positioned to benefit from regular access and routinemaintenance, others may be unsuitable for maintenance either because ofdifficult access, highly distributed arrangements, or excessive costs.In a variety of applications, limits on maintenance reflect a vastinstallation. Consider, for example, a border security application inwhich thousands of sensors are arrayed along hundreds of miles offrontier. In principle, an access road can be built and regularmaintenance can be scheduled. In practice, there may be sections inwhich road access is problematic and the sensor array may be so vastthat dedicated maintenance crews may have to be assigned continuously.

Another example is rural broadband access to the Internet or World WideWeb (WWW), in which one approach is a dispersed array of pole-mountedrepeaters. If these repeaters can be set up with an “install and forget”strategy, service providers can substantially function without dedicatedmaintenance crews.

These unattended electrically-operated systems typically utilize energystorage devices or units, to maintain power availability at night, tohelp maintain operation through intervals of bad weather, and to allowthe electrical load to draw power in short-term bursts that might exceedthe delivery capability of their energy recharge unit generation orrecharge units. Moreover, these unattended electrically-operated systemsgenerally need power conversion and regulation to deliver reliable,consistent power independent of conditions on the energy recharge unitsor in the storage units.

In solar applications, conventional remote power systems usecombinations of solar panels for energy recharge units and rechargeablebatteries for energy storage units. Typically, batteries are connecteddirectly to an output, while the solar panels are connected eitherthrough a diode or through a switching power converter. The direct useof batteries typically limits the degree of output regulation and doesnot provide for the longest possible life of these unattendedelectrically-operated systems. Thus, the battery terminals serve as thedirect power output, in which case the only protection is a fuse. Assuch, the quality of output regulation is determined entirely by thebattery and will follow wide tolerances.

These unattended systems typically lack reliability as battery chargingprocesses are not properly managed. Overcharge and underchargeconditions can occur, especially during long periods of cloudy weather.Battery life is relatively limited as a result. If a short circuit blowsa fuse, the system will be down until serviced. Multiple battery unitsmay be interconnected, but there is no control mechanism for loadsharing or balancing. Further, multiple battery units may be connectedto a single output in a modular fashion, and protection and interactionbetween and among the batteries are not adequate. Thus, these unattendedsystems need to protect themselves as well as their batteries againstoutput short circuits and other external faults.

In some unattended systems, either the energy recharge unit or therechargeable storage unit is connected directly to a dc bus, and theother unit is connected through a dc-dc converter. As such, only onedc-dc converter is utilized while having independent control of theenergy recharge unit and the rechargeable storage unit regardless of theserviced load. This arrangement supports an improved integration ofrecharge and storage over the basic solar panel and batteryinterconnections, but still does not resolve regulation or protectionissues.

In other unattended systems, the energy recharge unit charges thestorage unit, which then charges a capacitor, which is then switchedinto the load. In this arrangement, power flow from the energy rechargeunit to the load goes through a series of device connections: the energyrecharge unit, the storage unit, the capacitor, and then the load. Thissequence of operations can result in extra power loss, especially duringintervals in which the power from the energy recharge unit iswell-matched to the load.

Therefore, a need exists for a modular system for unattended energygeneration and storage that overcomes the problems noted above andothers previously experienced for addressing issues of regulation,protection, interconnection, or modularity. These and other needs willbecome apparent to those of skill in the art after reading the presentspecification.

SUMMARY

The foregoing problems are solved and a technical advance is achieved bythe present invention. Articles of manufacture and systems consistentwith the present invention provide an apparatus or modular system forunattended energy generation and storage for supplying energy to a load.The apparatus includes an energy recharge unit, an energy storage unit,an energy converter connected to the energy recharge unit, the energyconverter being capable of transferring energy at a power level from theenergy recharge unit to an output node, the power level being determinedby a power transfer controller, and a bi-directional energy converterconnected to the energy storage unit and to the output node. Thebi-directional energy converter is capable of converting energy ofvarying voltages from the energy storage unit to energy of varyingcurrent levels to supplement the transferred energy with energy from theenergy storage unit so as to maintain a constant voltage on the outputnode. The bi-directional energy converter is capable of converting thetransferred energy to provide charging energy to the energy storage unitwhen the transferred energy exceeds a demand level of the load whilemaintaining the constant voltage at the output node.

Articles of manufacture consistent with the present invention alsoprovide a method of providing energy to a load from a power supplysystem. The power supply system includes an energy recharge unit, anenergy converter with its input connected to the energy recharge unitand its output connected to an output node and coupled to a powertransfer controller, a rechargeable energy storage unit, abi-directional energy converter with its input connected to therechargeable energy storage unit and its output connected to the outputnode, the output node being connected to an input of the load. Themethod exposes the energy recharge unit to a corresponding energy sourceso as to produce electric energy, determines via the power transfercontroller whether a power level of the produced electric energy isabove a predetermined power threshold in order to activate the energyconverter, converts the produced electric energy by the energy converterand delivering the converted electric energy to the load in order tomeet at least part of a demand level of the load, monitors a voltage atthe output node. The method further converts energy stored in the energystorage unit by the bi-directional energy converter to supplement thedelivery of the converted produced energy to the load so as to maintainthe output node voltage at a predetermined voltage level, and convertsthe transferred energy to provide charging energy to the energy storageunit when the transferred energy exceeds a demand level of the loadwhile maintaining the predetermined voltage level at the output node.

Other systems, apparatus, methods, features, and advantages of thepresent invention will be or will become apparent to one with skill inthe art upon examination of the following figures and detaileddescription. It is intended that all such additional systems, methods,features, and advantages be included within this description, be withinthe scope of the invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an implementation of the presentinvention and, together with the description, serve to explain theadvantages and principles of the invention. In the drawings:

FIG. 1 is a schematic diagram illustrating one embodiment of a modularunattended system for energy generation and storage consistent with thepresent invention;

FIGS. 2A and B are schematic diagrams illustrating the modularunattended system for energy generation and storage of FIG. 1 withprotection elements consistent with the present invention;

FIG. 3 is a flow chart illustrating a method for an initial start-up ofthe modular unattended system for energy generation and storage of FIG.1 consistent with the present invention; and

FIG. 4 is a flow chart illustrating a method for detecting andmitigating a fault disturbing an on-going operation of the modularunattended system for energy generation and storage of FIG. 1 consistentwith the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference will now be made in detail to an implementation consistentwith the present invention as illustrated in the accompanying drawings.Wherever possible, the same reference numbers will be used throughoutthe drawings and the following description to refer to the same or likeparts.

FIG. 1 depicts one embodiment of a modular system for unattended energygeneration and storage or power supply apparatus 100 consistent with thepresent invention. The power supply apparatus 100 is a modular systemthat comprises an energy recharge device or unit 10 suitable forlong-term operation and expected to provide variable energy, an energystorage device or unit 12 which is charged and recharged by the energyrecharge unit 10, a switching power conversion circuit 14 that drawspower when activated from the energy recharge unit 10 in a manner thataddresses the needs of a load 16. The switching power conversion circuit14 will be hereafter referred to as the recharge unit or sourceconverter 14. The power supply apparatus 100 further comprises anotherswitching power conversion circuit 18 that manages the energy storageunit 12 and maintains an output suitable for the variable or arbitraryelectrical load 16. The switching power conversion circuit 18 will behereafter referred to as the storage converter 18.

The power supply apparatus 100 is configured to enable energy deliveryto the load 16 from the energy recharge unit 10 while also providingenergy to the energy storage unit 12 so as to augment, if necessary, theenergy stored in the energy storage unit 12 when sufficient rechargeunit energy is available. The power supply apparatus 100 is alsoconfigured to enable energy delivery to the load 16 from the energyrecharge unit 10 and/or from the energy storage unit 12 when the energyrecharge unit 10 is unable to meet the energy needs of the load 16. Thepower supply apparatus 100 also prevents energy delivery to the load 16when the charge level of the energy storage unit 12 falls below aminimum voltage threshold level so that all of the energy generated bythe energy recharge unit 10 is supplied or made available to the energystorage unit 12 until the voltage of the energy storage unit 12 reachesthis minimum voltage threshold level at which the storage converter 28will operate at a minimum desired energy conversion efficiency level. Assuch, the energy recharge unit 10 can build up the amount of energy inthe energy storage unit 12 first and then to deliver and maintaindelivery of energy to the load 16 as well as to continue building up andmaintaining energy in the storage energy unit 12 during periods when theenergy recharge unit 10 is producing enough energy to do so.

The energy recharge unit 10 may be any kind of power or energygenerating system, such as for example solar panel, solar cell, fuelcell, wind generator, rotary generator, etc. For the purposes of thepresent invention, the energy recharge unit 10 is a solar panel. Oneimportant feature is that the energy recharge unit 10 is configured tobe controlled to maximize its performance and longevity. In the case ofa solar or wind recharge unit, the control draws maximum available powerwhenever power is available and useful. In the case of a fuel cell orrotary generator, the control acts to maintain operation at an optimumpower point, selected for the specific technology. The preferredapplication is an unattended long-term power supply, so those energysystems that require substantially no maintenance are most suitable.These would include solar photovoltaic panels, wind generators, smallwater wheels, or devices able to extract energy from the immediatesurroundings. Almost all plausible energy recharge units 10 for thispurpose have an identified optimum operating condition. As such, theenergy recharge unit 10 uses a switching power converter controlled toenforce this optimum. This is intended to make the operation of theenergy recharge unit 10 nearly independent from those of the load 16 andthe energy storage unit 12.

In the preferred embodiment, the energy storage unit 12 is based onenergy storage elements 26 such as ultracapacitors. Among electrical andelectrochemical storage devices, ultracapacitors 26 are known to providethe highest number of charge and discharge cycles and to have long lifeas well. Other energy accumulators, especially secondary batteries butalso devices such as flywheels or other rechargeable mechanical devicesare applicable. Ultracapacitors 26 or batteries or other rechargeableelements may be used in series strings owing to their relatively lowterminal voltages. To ensure long life, the individual storage elementsin the series string need to be matched in voltage. Known balancingtechnology provides this function, which in turn assures the longestpossible life. In ultracapacitors and certain battery types, notablylithium-ion rechargeable cells, life is further extended by limiting thecharge voltage. Balancing technology allows the voltage to be set to anydesired value, and can maintain balance even if a decreased voltage isdesired. In the unattended power supply apparatus 100, the energystorage unit 12 may be controlled to permit operation even when theenergy recharge unit 10 is unavailable, and is protected against bothunder-voltage and over-voltage to prevent failures.

The load 16 can be any kind of electric load, which requires electricpower at a rate or magnitude that does not exceed the capacities of theenergy storage unit 12 and the energy recharge unit 10 over time, or forwhich periods of nonuse during shortages of such capacities can betolerated before the energy recharge unit 10 is re-energized andself-started. The energy recharge unit 10 and the energy storage unit 12need to be suitable for the load 16 that may require either fairlysteady or intermittent current. Moreover, the energy storage unit 12needs to be able to accumulate enough energy capacity to power the load16 for some periods of time during which the energy recharge unit 10 isincapable of doing so, for example, during the dark of night or duringcloudy weather when the energy recharge unit 10 is a solar panel.

Now referring back to FIG. 1, the energy recharge unit 10 is connectedto the source converter 14 such that energy or power generated orotherwise produced by the energy recharge unit 10 flows through thesource converter 14 to the load 16 and to the storage converter 18 viathe output node 24. The source converter 14 is a dc-dc switching powerconverter, and preferably a boost type dc-dc converter. The sourceconverter 14 incorporates a maximum power point tracker (MPPT) or powercontroller 26, which serves to insure that the energy recharge unit 10generates maximum power without regard to voltage at output node 24 orother conditions. When the energy recharge unit 10 is a solar panel, thesource converter 14 (and its MPPT controller 26) functions only whensolar energy is available, such as during daylight hours. That is, aninternal processor (not shown) of the source converter 14 does notconsume energy unless solar energy is available. This operationalfeature of the source converter 14 is accomplished by supplying therecharge unit converter controls from the panel of the energy rechargeunit 10.

By incorporating the MPPT controller 26, the source converter 14 becomesa variable dc-dc converter that may use a control algorithm to force theterminals of the photovoltaic panel to an impedance that produces themaximum power out of the panel of the energy recharge unit 10. The MPPTcontroller 26 adjusts the panel operating point to extract maximum poweron a moment-by-moment basis. This MPPT controller 26 may act entirelybased on panel terminal characteristics so as to function irrespectivelyof the voltage at output node 24. As such, the source converter 14 doesnot disrupt voltage level or interfere with voltage regulation actionelsewhere in the power supply apparatus 100. Many MPPT methods can beused, such as ripple correlation control, perturb-and-observe approachesor constant-voltage-fraction approaches.

The MPPT controller 26 is also configured to enforce a voltage limit andpermit an external digital command to shut it down. Without such a limitor command, excess solar power may continue to be delivered from theenergy recharge unit 10 even when the load 16 is light and the storageenergy unit 12 is at full capacity. In this situation, solar energy isnot needed, and there is a potential for overcharging the energy storageunit 12 or delivering excessive voltage at the output node 24. Moreover,when the produced energy falls below a predetermined energy level, theMPPT controller deactivates the source converter 14. Power trackingsubject to a voltage limit and shutdown command is a known practice buthas not been used previously with a separate parallel converter, such asthe storage converter 18, which manages a different function.

As stated earlier, the energy storage unit 12 is connected to the outputnode 24 through the storage converter 18. The storage converter 18 is abi-directional dc-dc boost converter configured to deliver a consistentor approximately constant output voltage level on the output node 24,regardless of its input voltage level. That is, even if the inputvoltage to the storage converter 18 varies, the output voltage of thestorage converter 18 on the output node 24, which is supplied to theload 16, preferably remains approximately constant, which may berequired by the load 16, especially if the load 16 includes amicroprocessor or other logic controller that typically requires such aconstant voltage input to avoid inconsistent or incorrect operation thatmay be caused by voltage fluctuations.

Typically, dc-dc voltage converters by nature have different conversionefficiency characteristics for different input voltages. At a low inputvoltage, the storage converter 18 may be so inefficient that it maydrain all the power available from the energy storage unit 12 at a ratefaster than the energy storage unit 12 and the energy recharge unit 10can supply and still not be able to deliver the required constantvoltage on the output node 24. In that condition, the power supplyapparatus 100 may not power the load 16. Therefore, the storageconverter 18 is activated only when its input voltage is high enough tooperate efficiently and that the energy available from the energystorage unit 12 is sufficient for the storage converter 18 to deliverthe required energy at the required constant voltage on output node 24to operate the load 16. As such, the storage converter 18 is activatedor controlled using a sensorless current mode (SCM) controller 20, withits primary objective being output voltage regulation. The SCMcontroller 20 is connected to the output of the energy storage unit 12and to the output of the storage converter 18. Alternately, the SCMcontroller 20, which may include a processor programmed with operationalinstructions, may be incorporated into the storage converter 18.

In SCM control, an inductor current is reconstructed from voltageinformation. The inductor voltage in a dc-dc converter is typically asubstantially larger signal than an output of a current sensor and itsrange does not change as a function of loading. In its simplest form,the SCM approach reconstructs the inductor current directly byintegrating the inductor voltage. Alternately, current limiting can beenforced by means of a separate current sensor. As such, current can belimited from the energy storage unit 12 to limit an energy flow when theoutput node 24 is short-circuited or overloaded. Other control methods,such as voltage-mode or current-mode controls, can be used if they areproperly designed to reject energy variations, although SCM has knownadvantages based on noise rejection and wide operating range.

Using the SCM controller 20, the storage converter 18 can be controlledto produce a substantially constant voltage at the output node 24. Ineffect, the storage converter 18 can deliver whatever current is neededto maintain the substantially constant voltage. If the current of theenergy recharge unit 10 exceeds the current amount required by the load16, the storage converter 18 takes the excess and acts to deliver it tothe energy storage unit 12 at the actual capacitor voltage level. If thecurrent of the energy recharge unit 10 is insufficient for the load 16,the storage converter 18 acts to take current from the energy storageunit 12 and to maintain the load with constant voltage output. Thus, thestorage converter 18 manages the current to take on whatever value isneeded (positive or negative) at any given moment to maintain theconstant voltage at output node 24 and support the load 16.

Moreover, the SCM controller 20 can monitor energy buildup in the energystorage unit 12 and detects when the voltage in the energy storage unit12 reaches or exceeds a minimum threshold level. More specifically, theSCM controller 20 may prevent any energy generated by the energyrecharge unit 10 and any energy in energy storage unit 12 from beingdelivered to the load 16 until such time as the energy storage unit 12has reached a minimum threshold of energy storage or a minimum chargelevel.

As stated above, in FIG. 1 the energy storage unit 12 uses theultracapacitors 26, which can deliver extreme cycle counts, 100,000 ormore, thus supporting decades of daily or even more frequent cycles.These ultracapacitors 26 have limited voltage ratings, typically 2.3 to2.7 V each, so they are used in series combinations in most storageapplications. In series combinations, a charge imbalance is reflected asa voltage imbalance, and it is imperative to make sure the highestvoltage in the series string stays below the operating limit.Furthermore, life extension can be obtained if an even lower voltagelimit is enforced. Similar limitations based on voltage apply tobatteries, which can be used in place of ultracapacitors 26 with onlyminor circuit modifications.

In the embodiment of FIG. 1, the energy storage unit 12 is coupled to anequalizer 22. The equalizer 22 can guarantee that each of theultracapacitors 26 has a corresponding voltage that is a correctfraction of the total string voltage. Thus, if there are 5ultracapacitors 26 in series and the total voltage is 10.0 V, then eachindividual ultracapacitor 26 is within a few millivolts of 2.0 V. Theuse of the equalizer 22 enables the monitoring and enforcement ofcapacitor voltage limits and the management of the energy storage unit12. The inclusion of the equalizer 22, especially to facilitatereduced-voltage operation, is an advantageous aspect of the invention.

As shown in FIG. 1, the power supply apparatus 100 can be subdividedinto two subsystems 11 and 13 and the load 16. Namely, an energyrecharge unit or recharge subsystem 11 which includes the energyrecharge unit 10 and the source converter 14 with its MPPT controller26, and an energy storage subsystem 13 which includes energy storageunit 12, the equalizer 22 and the storage converter 18 with its SCMcontroller 20.

An advantageous feature of the power supply apparatus 100 is that thesystem-level power converter control, which includes controlsincorporated into the source converter 14 and the storage converter 18,decouples the action of the individual subsystems 11 and 13. This allowsthe two subsystems 11 and 13 to interact in a simple and parallel mannerwithout introducing control challenges. The basic action is that theenergy recharge unit 10 is controlled for power flow, the output node 24is controlled for fixed voltage, and the current to and from the energystorage unit 12 is controlled to make up any differences.

As both the energy recharge subsystem 11 and the energy storagesubsystem 13 supply the load 16 in parallel, the solar energy produceddoes not go through multiple power processing stages on its way to theload 16. Moreover, as the output node 24 need not be connected directlyeither to the energy storage unit 12 or to the energy recharge unit 10,each of the intervening dc-dc converters 14 and 18 manages the needs ofthe corresponding subsystems 11 and 13 while delivering regulated outputpower to the output node 24 that serves the load 16. This arrangement ofthe power supply apparatus 100 can be termed a “virtual bus” structure.

Now referring to FIGS. 2A and B, another embodiment of the power supplyapparatus 100 of FIG. 1 is illustrated with protection elementsconsistent with the present invention. As shown, the subsystem 11 nowincludes an inductor element 30 connected at the output of the energyrecharge unit 10, a unidirectional element 32, such as a diode,connected in series between the inductor element 30 and the load 16, anda switching element q₄ 34 connected in parallel with the energy rechargeunit 10 and the inductor 30. The switching element q₄ 34, which can alsobe referred as the recharge unit switch q₄ 34, is coupled for switchingoperations to the source converter 14.

As shown in FIGS. 2A and B, the subsystem 13 includes an inductorelement 36 connected at the output of the storage unit 12, and aswitching element q₁ 38 connected in parallel with the storagerechargeable unit 12 and the inductor element 36. The switching elementq₁ 38 may be a field-effect transistor (FET) or metal-oxidesemiconductor FET (MOSFET). The recharge unit switch q₁ 38, is coupledfor switching operations to the storage converter 18. Additionally, asshown in FIG. 2A two other switching elements q₂ 40 and q₃ 42 areconnected in series between the inductor element 36 and the output node24, and coupled for switching operations to the storage converter 18.The switching elements q₂ 40 and q₃ 42 may be field-effect transistors(FET) or metal-oxide semiconductor FETs (MOSFET). Alternately, theswitching elements q₃ 42 may be positioned between the output node 24and the load 16, as shown in FIG. 2B. Moreover, a current sensor 44 isused to supply a value of a current I_(bat) of the energy storage unit12 to the storage converter 18.

Now referring to FIG. 3, a flow chart 300 illustrates a method for aninitial start-up of the power supply apparatus 100 of FIG. 1 consistentwith the present invention. The energy storage converter 18 isconfigured with its SCM controller 20 to manage the start up procedure.At an initial start-up of the power supply apparatus 100, the energyrecharge unit 10 being a solar cell is exposed to available sunlight instep 302, and the energy storage unit 12 is in this initial conditionhas no stored energy charge or only a very low amount of stored energyor charge. Upon initial start up of the power supply apparatus 100 inthis configuration, the storage converter 18 prevents any energyconverted by the source converter 14 from being delivered to the load16, at step 304. That is, the storage converter 18 ensures that all ofthe energy generated by the energy recharge unit 10 is flowing to theenergy storage unit 12, at step 306. The storage converter 18 managesthis power flow and prevents excessive starting current by means of ahardware limit on the duty ratio of recharge unit switch q₁ 38. Once theenergy storage unit 12 reaches a minimum threshold for its energy level,at step 308, the storage converter 18 allows the energy generated by theenergy recharge unit 10 to be delivered to the load 16, at step 310,while a remainder of the energy generated, if any, is supplied to theenergy storage unit 12 so that the energy level in the energy storageunit 12 continues to increase. That is, once the voltage of the energystorage unit 12 has exceeded the minimum allowed threshold (minimumoperating energy level), then the power supply apparatus 100 may supplythe load 16 (i.e. enter normal operation), at step 312. Moreover, asstated above during normal operation the energy storage unit 12 cansupply the energy required by the load 16 when the energy recharge unit10 cannot generate sufficient energy to operate the load 16.

Moreover, when the energy storage unit 12 is discharged to below theminimum allowed operating energy level, that is, its terminal voltage islow, The recharge unit switch q₁ 38 is turned off and a diode (notshown) allows energy to flow into but not out of the energy storage unit12. Conversely, when the energy storage unit 12 is completely charged(maximum allowed operating energy level), that is, its terminal voltageis high, the storage converter 18 sends a disable signal to shut off thesource converter 14. In this circuit condition, the switching element q₂40 is also turned off and the diode (not shown) allows only energy toflow out of but not into the energy storage unit 12.

Now referring to FIG. 4, a flow chart 400 illustrates a method fordetecting and mitigating a fault disturbing an on-going operation of thepower supply apparatus 100 of FIG. 1 consistent with the presentinvention. As shown in FIGS. 2A and B, the output protection switchingelement q₃ 42, which may be provided in alternate positions, allows theenergy storage subsystem 13 to be completely disconnected if a problemoccurs. This is an unusual arrangement in a dc-dc converter and may be aunique aspect in this particular application. Further, an automaticrecovery procedure programmed in the processor of the storage converter18 to restore power if a short-term output problem appears during normaloperation, at step 402, is described below.

If a short circuit is detected occurring at the output node 24, at step404, the protection switch q₃ 42 disconnects immediately, at step 406.Switch ratings are chosen so the protection switch q₃ 42 can interrupt amaximum short-circuit current. As such, the energy stored in theultracapacitors 26 will not dissipate into the short circuit. When thisswitch q3 42 is open the storage subsystem 13 can charge when sufficientvoltage is available, at step 408, because a reverse parallel diode (notshown) connected with switch q₃ 42 permits inflow but not outflow. Thestorage subsystem 13 may not discharge until a fault recovery sequenceis completed. The protection switch q3 42 also opens if the capacitorvoltage falls below a lower limit or threshold, indicating possibleundercharge so as to protect the energy storage subsystem 13 againsttotal energy loss if a fault occurs. Energy remains available, andallows the power supply system apparatus 100 to recover normal operationimmediately when the fault is gone and the energy recharge unit 10 isonce again delivering power. After determining if a predetermined amountof time has elapsed, at step 410, and once the fault clears, the storageconverter 18 enters a recovery mode. At step 412, the switch q₃ 42 isturned on and the storage converter 18 begins providing energy to theoutput node 24. At step 414, a check is made as to whether the fault isstill present. In the affirmative, switch q₃ 42 turns off again.Otherwise, the output voltage at output node 24 starts to come up andnormal operation resumes, at step 416.

Another advantageous protection feature of the invention is thepositioning of the unidirectional element or diode 32 in the rechargeunit subsystem 11. This diode 32 is provided at an output of the energyrecharge unit 10 to prevent current from flowing toward it. That is, thediode 32 permits current outflow but not inflow toward the energyrecharge unit 10. In this embodiment of the power supply apparatus 100,no additional protection switching element is provided between theenergy recharge unit or solar panel 10 and the load 16, because solarpanels are inherently current limited. That is, a short-circuit currentis typically only slightly higher than the rated output current of thepower supply apparatus 100. As such, short circuit conditions do notresult in excessive current flows, and the source converter 14 and itsMPPT controller 26 may not need to be shut-off because as discussedabove a separate process programmed in the storage converter 18 managesthe recovery process from a short circuit condition.

In summary, the combination of power-based tracking control for thesource converter 14 and SCM control for the storage converter 18 is anadvantageous feature of the invention. These two controls work togetherin a highly useful manner. The MPPT controller 26 acts to maximize powerfrom the energy recharge unit 10 no matter what voltage is present atthe output node 24. The SCM controller 20 acts to regulate the voltageno matter what the power. As a result, undesired dynamic interactionsare avoided. When the energy recharge unit 10 is able to deliver extrapower, this power flows into the energy storage unit 12 without anyextra intentional effort. When the energy recharge unit 10 isinsufficient or absent, energy flows from the energy storage unit 12 tomaintain the desired voltage at the output node 24. The two controls canbe designed independently, and work together in a manner that is highlybeneficial for the modular arrangement of the power supply apparatus100. The only added aspect needed to avoid undesired interactions is toaddress the condition when the energy storage unit 12 reaches its “full”state or charge. In this case, extra power input to this modular systemis not useful. Under this condition, which is sensed easily byconfirming that the energy storage unit 12 has reached its voltagelimit, the energy storage converter 18 sends a signal to shut off thesource converter 14. The energy recharge unit 10 will not turn back onuntil its power is again useful to the modular system or power supplyapparatus 100. As such, the energy storage unit 12 is managed with bothupper and lower voltage limits to prevent overcharge and undercharge. Alimited charge range is essential for high reliability. A current limitis enforced both on the energy storage subsystem 11 and at the outputnode 24. If the output current becomes excessive, then the protectiondevice q₂ 40 temporarily shuts off the output.

This control combination is advantageous because it delivers excellentoutput regulation no matter how power is flowing in the power supplysystem apparatus 100. Since the power flow is uncertain and highlyvariable, it is essential that the regulation function be carried out ina manner independent of flows, directions, and other factors. The SCMcontroller 20 is able to carry out its function even when short-termload peaks are applied to the power supply system apparatus 100. Anadvantage is that highly variable loads that require intermittent powercan be handled without special problems with this power supply systemapparatus 100. Loads with consistent, constant power needs are alsohandled without trouble.

While various embodiments of the present invention have been described,it will be apparent to those of skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. Accordingly, the present invention is not to berestricted except in light of the attached claims and their equivalents.

1. An apparatus for supplying energy to a load, comprising: an energyrecharge unit; an energy storage unit; an energy converter connected tosaid energy recharge unit, said energy converter being capable oftransferring energy at a power level from said energy recharge unit toan output node at a predetermined voltage, said power level beingdetermined by a power transfer controller; and a bi-directional energyconverter connected to said energy storage unit and to said output node;a sensorless current mode (SCM) controller connected to the energystorage unit to receive an output of the energy storage, to the outputnode to receive an output of the bi-directional energy converter, and tothe bi-directional energy converter, the SCM controller being configuredto control the operation of the bi-directional energy converter, basedon the output of the energy storage and the output of the bi-directionalenergy converter, to maintain the voltage of the output node at thepredetermined voltage by: (i) converting energy of varying voltages fromsaid energy storage unit to energy of varying current levels at thepredetermined voltage to supplement said transferred energy with energyfrom said energy storage unit, (ii) converting said transferred energyto provide charging energy to said energy storage unit when saidtransferred energy exceeds a demand level of said load, and (iii)deactivating the bi-directional energy converter in response to thetransferred energy falling below the demand level of said load and acharge level of the energy storage unit falling below a predeterminedlower charge threshold.
 2. The apparatus of claim 1, wherein saidbi-directional energy converter interrupts said charging of said energystorage unit when a charge level of said energy storage unit reaches apredetermined upper charge threshold.
 3. The apparatus of claim 1,wherein said bi-directional energy converter maintains said constantvoltage at said output node under varying input voltages.
 4. Theapparatus of claim 1, wherein said power transfer controller determinessaid power level of said transferred energy by optimizing said producedelectric energy by adjusting an operating point of said energy rechargeunit.
 5. The apparatus of claim 1, wherein said energy converter isdeactivated when said produced energy falls below a predetermined energylevel.
 6. The apparatus according to claim 1, further comprising aprotection element that serves to disconnect said energy storage unitwhen a fault is detected at said output node.
 7. The apparatus accordingto claim 6, wherein said protection element reconnects said energystorage unit when said fault has been cleared, and said bi-directionalenergy converter resumes supplementing said transferred energy withenergy from said energy recharge unit.
 8. The apparatus according toclaim 1, further comprising a unidirectional conducting element providedat an output of said energy recharge unit to prevent current fromflowing toward said energy recharge unit.
 9. The apparatus according toclaim 1, wherein said energy storage unit comprises a plurality ofrechargeable elements with a capability of receiving energy from saidenergy recharge unit.
 10. The apparatus according to claim 9, furthercomprising an equalizer that serves to ensure a voltage balance and acharge balance among said plurality of rechargeable elements.
 11. Theapparatus according to claim 9, wherein said plurality of rechargeableelements comprises batteries.
 12. The apparatus according to claim 9,wherein said plurality of rechargeable elements comprises capacitors.13. The apparatus according to claim 12, wherein said capacitors areultra-capacitors.
 14. The apparatus of claim 1, wherein said energyconverter is a dc-dc converter.
 15. The apparatus of claim 1, whereinsaid bi-directional energy converter is a dc-dc bi-directionalconverter.
 16. The apparatus of claim 1, wherein said energy rechargeunit is one of a solar power unit, a wind power unit, a flywheel powerunit, a geothermal power unit and a generator unit.
 17. A method ofproviding energy to a load from a power supply system, said power supplysystem comprising an energy recharge unit, an energy converter with aninput connected to the energy recharge unit and an output connected toan output node and coupled to a power transfer controller, arechargeable energy storage unit, a bi-directional energy converter withan input connected to said rechargeable energy storage unit and anoutput connected to said output node, said output node being connectedto an input of said load, and a sensorless current mode (SCM) controllerwith a first input connected to said rechargeable energy storage unitand a second input connected to the output of the bi-directional energyconverter, said method comprising: exposing said energy recharge unit toa corresponding energy source as to produce electric energy; determiningvia said power transfer controller whether a power level of saidproduced electric energy is above a predetermined power threshold inorder to activate said energy converter; converting said producedelectric energy by said energy converter and delivering said convertedelectric energy to said load at a predetermined voltage in order to meetat least part of a demand level of said load; monitoring a voltage atsaid output node; and controlling the operation of the bi-directionalenergy converter with the SCM controller, based on an output of theenergy storage unit and an output of the bi-directional energyconverter, to maintain the voltage of said output node at thepredetermined voltage by: (i) converting energy stored in said energystorage unit by said bi-directional energy converter to supplement saiddelivery of said converted produced energy to said load, and (ii)converting said transferred energy to provide charging energy to saidenergy storage unit when said transferred energy exceeds the demandlevel of said load, and (iii) deactivating the bi-directional energyconverter in response to the transferred energy falling below the demandlevel of said load and a charge level of the energy storage unit fallingbelow a predetermined lower charge threshold.
 18. The method accordingto claim 17, further comprising: deactivating said energy converter whensaid power level of said produced electric energy falls below saidpredetermined power threshold.
 19. The method according to claim 17,further comprising: deactivating said energy converter when said demandlevel of said load falls below a predetermined load threshold and acharge level of said energy storage unit is above a predetermined chargethreshold.
 20. The method according to claim 17, further comprising:delivering said produced electric energy to said bi-directional energyconverter to charge said energy storage unit when a charge level of saidenergy storage unit is below a predetermined charge upper threshold. 21.The method according to claim 20, further comprising: discontinuingcharging said energy storage unit when said charge level of said energystorage unit is at or near said predetermined charge upper threshold.22. The method according to claim 17, further comprising: preventingdelivery of said converted electric energy to said load when a chargelevel of said energy storage unit is below a predetermined charge lowerthreshold; delivering said produced electric energy to saidbi-directional energy converter to charge said energy storage unit untilsaid charge level of said energy storage unit is above saidpredetermined charge lower threshold; and resuming delivery saidconverted electric energy to said load.
 23. The method according toclaim 17, further comprising: optimizing said produced electric energyby adjusting an operating point of said energy recharge unit via saidpower transfer controller.
 24. The method according to claim 17, furthercomprising: disconnecting said energy storage unit when a fault isdetected at said output node.
 25. The method of claim 17, wherein saidenergy recharge unit is one of a solar power unit, a wind power unit, aflywheel power unit, a geothermal power unit and a generator.
 26. Themethod of claim 17, wherein said energy converter is a dc-dc converter.27. The method of claim 17, wherein said energy recharge unit comprisesa plurality of rechargeable elements.
 28. The method according to claim27, wherein said plurality of rechargeable elements comprises batteries.29. The method according to claim 27, wherein said plurality ofrechargeable elements comprises capacitors.
 30. The method according toclaim 29, wherein said capacitors are ultra-capacitors.
 31. The methodaccording to claim 17, further comprising: balancing a voltage balanceand a charge balance of said plurality of rechargeable elements via anequalizer.
 32. The method of claim 17, wherein said bi-directionalenergy converter is a dc-dc bi-directional converter.
 33. A computerreadable medium comprising instructions which when executed by acomputer system causes the computer to implement a method for providingenergy to a load from a power apparatus, said power apparatus comprisingan energy recharge unit, an energy converter with an input connected tothe energy recharge unit and coupled to a power transfer controller, arechargeable energy storage unit, a bi-directional energy converter withan input connected to said rechargeable energy storage unit, and asensorless current mode (SCM) controller connected to said rechargeableenergy storage unit and to the bi-directional energy converter, saidmethod comprising: exposing said energy recharge unit to a correspondingenergy source so as to produce electric energy; determining via saidpower transfer controller whether a power level of said producedelectric energy is above a predetermined power threshold in order toactivate said energy converter; converting said produced electric energyby said energy converter and delivering said converted electric energyto said load at a predetermined voltage in order to meet at least partof a demand level of said load; monitoring a voltage at said outputnode; and controlling the operation of the bi-directional energyconverter with the SCM controller, based on an output of the energystorage unit and an output of the bi-directional energy converter, tomaintain the voltage of said output node at the predetermined voltageby: (i) converting energy stored in said energy storage unit by saidbi-directional energy converter to supplement said delivery of saidconverted produced energy to said load, (ii) converting said transferredenergy to provide charging energy to said energy storage unit when saidtransferred energy exceeds the demand level of said load, and (iii)deactivating the bi-directional energy converter in response to thetransferred energy falling below the demand level of said load and acharge level of the energy storage unit falling below a predeterminedlower charge threshold.
 34. A system for performing a method forproviding energy to a load from a power apparatus, said power apparatuscomprising an energy recharge unit, an energy converter with an inputconnected to the energy recharge unit and coupled to a power transfercontroller, a rechargeable energy storage unit, a bi-directional energyconverter with an input connected to said rechargeable energy storageunit, said energy recharge unit being exposed to a corresponding energysource so as to produce electric energy, the system comprising: at leastone processor programmed to determine via said power transfer controllerwhether a power level of said produced electric energy is above apredetermined power threshold in order to activate said energyconverter, to activate said energy converter to convert said producedelectric energy and to deliver said converted electric energy to saidload at a predetermined voltage in order to meet at least part of ademand level of said load; and a sensorless current mode (SCM)controller connected to the rechargeable energy storage unit to receivean output of the energy storage, to the output node to receive an outputof the bi-directional energy converter, and to the bi-directional energyconverter, the SCM controller being configured to control the operationof the bi-directional energy converter, based on the output of theenergy storage and the output of the bi-directional energy converter, tomaintain the voltage of said input at the predetermined voltage by: (i)converting energy stored in said energy storage unit for supplementingsaid delivery of said converted produced energy to said load (ii)converting at least part of said transferred energy to provide chargingenergy to said energy storage unit when said transferred energy exceedsthe demand level of said load, and (iii) deactivating the bi-directionalenergy converter in response to the transferred energy falling below thedemand level of said load and a charge level of the energy storage unitfalling below a predetermined lower charge threshold.
 35. The systemaccording to claim 34, further comprising: at least one processorprogrammed to deactivate said energy converter when said power level ofsaid produced electric energy falls below said predetermined powerthreshold.
 36. The system according to claim 34, further comprising: atleast one processor programmed to deactivate said energy converter whensaid demand level of said load falls below a predetermined loadthreshold and a charge level of said energy storage unit is at or near apredetermined upper charge threshold.
 37. The system according to claim34, further comprising: at least one processor programmed to discontinuedelivery of said produced electric energy to said load when said demandlevel of said load falls below a predetermined load threshold, and todeliver said produced electric energy to said bi-directional energyconverter to charge said energy storage unit when a charge level of saidenergy storage unit is below a predetermined charge upper threshold. 38.The system according to claim 34, further comprising: at least oneprocessor programmed to discontinue charging said energy storage unitwhen said charge level of said energy storage unit is at or above saidpredetermined charge upper threshold.
 39. The system according to claim34, further comprising: at least one processor programmed to preventdelivery of said converted electric energy to said load when a chargelevel of said energy storage unit is below a predetermined charge lowerthreshold, to deliver said converted electric energy to saidbi-directional energy converter to charge said energy storage unit untilsaid charge level of said energy storage unit is above saidpredetermined charge lower threshold.
 40. The system according to claim34, further comprising: at least one processor programmed to optimizesaid produced electric energy by adjusting an operating point of saidenergy recharge unit via said power transfer controller.
 41. The systemaccording to claim 34, further comprising: at least one processorprogrammed to disconnect said energy storage unit when a fault isdetected at said output node.